Various types of dot matrix line printers have been proposed and are in use. In general, dot matrix line printers include a print head comprising a plurality of dot printing mechanisms, each including a dot forming element. The dot forming elements are located along a line that lies orthogonal to the direction of paper movement through the printer. Since paper movement is normally vertical, the dot forming elements usually lie along a horizontal line. Located on the side of the paper remote from the dot forming elements is a platen and located between the dot forming elements and the paper is a ribbon. During printing, the dot forming elements are actuated to create one or more dots along the print line defined by the dot forming elements. The paper is incremented forwardly after each dot row is printed. A series of dot rows creates a row of characters.
In general, dot matrix line printers fall into two categories. In the first category are dot matrix line printers wherein only the dot forming elements are shuttled. In the second category are dot matrix line printers wherein the entire print head, e.g., the actuating mechanism as well as the dot forming elements are shuttled. Regardless of type, the portions of the dot printing mechanisms to be shuttled are mounted on or form a carriage and the carriage is reciprocated back and forth (e.g., shuttled) by a shuttling mechanism. The present invention is useful with both catagories of dot matrix printers. More specifically, while the invention was developed for use in connection with a dot matrix line printer wherein the entire print head is shuttled, the invention can also be utilized with dot matrix line printers wherein only the dot forming elements are shuttled.
In the past, both types of dot matrix line printers, i.e., those wherein only the dot forming elements are shuttled and those wherein the entire print head is shuttled, have been supported by flexures. In most instances, the items to be shuttled are supported by a pair of flexures each formed of an elongate piece of flat spring steel. One end of the flat spring steel piece is attached to the frame of the printer and the other end is attached to the carriage that supports the items to be shuttled. The shuttle drive mechanism of the invention is designed for use with flexure mounted carriages, particularly flexure mounted carriages that support the entire print head of a dot matrix line printer.
In the past, various types of carriage shuttling mechanisms have been proposed to shuttle the flexure supported items of dot matrix line printers of the type described above. One type of carriage shuttling mechanism includes a stepping motor that is connected to the carriage so as to cause step increments of carriage movement. At the end of each step, the appropriate actuating mechanisms are energized to create dots. Bi-directional printing is provided by stepping the carriage first in one direction and then in the opposite direction. A major disadvantage resulting from the use of stepping motors in dot matrix line printers, particularly dot matrix line printers wherein the actuating mechanisms as well as the dot forming elements are shuttled, is that conventionally sized stepping motors have insufficient power to shuttle the print head of such dot matrix line printers. That is, while conventionally sized stepping motors have adequate power to shuttle only the dot forming elements, they are marginal at best in printers wherein the entire print head is shuttled. In addition, stepping motors have a speed limitation that makes them undesirable for use in relatively high speed dot matrix line printers, e.g., 600 and above lines per minute (lpm) dot matrix line printers.
As a result of the inherent limitations of stepping motor shuttle systems, attempts have been made to utilize constant speed AC and DC motors to shuttle the movable items of the print head of dot matrix line printers. One of the major disadvantages of constant speed motor shuttling systems resides in the coupling mechanism used to couple the motors to the carriage that supports the items to be shuttled. In most instances, the coupling mechanism is a cam and cam follower mechanism. Cam/cam follower mechanisms are undesirable in dot matrix line printer shuttle systems because they are subject to mechanical wear. More specifically, dot matrix line printers, particularly in high speed dot matrix line printers, require precision positioning of the dot forming elements at the time they are actuated by their related actuating mechanisms. Mechanical wear is highly undesirable becauses it reduces the precision with which the dot forming elements can be positioned. As positioning precision drops, dot misregistration increases. As a result, printed characters and images are distorted and/or blurred. Distorted and/or blurred images are, of course, unacceptable in environments where high quality printing is required or desired.
Another disadvantage of many prior art carriage shuttling systems that include constant speed motors and cam/cam follower coupling mechanisms is that the displacement vs. time curve that they produce is nonlinear. As a result, relatively sophisticated carriage position sensing and control systems are required if precise dot positioning is to be achieved.
In order to minimize the mechanical wear factor and nonlinear carriage displacement vs. time curve problems produced by prior systems for mechanically coupling a constant speed motor to the print elements of a dot matrix line printer, a proposal has been made to use a coupling system that includes a pair of eliptical pulleys. See U.S. Pat. No. 4,387,642 entitled "Bi-Directional, Constant Velocity, Carriage Shuttling Mechanisms" by Edward D. Bringhurst et al. While the bi-lobed second order eliptical gear coupling mechanism described in this patent application has certain advantages over prior coupling mechanisms, it also has certain disadvantages. For example, it is undesirably noisy, mechanically complex and more expensive to manufacture than desirable.
In addition to stepping motor systems and constant speed motor systems, in the past, linear motors have been used to shuttle the carriages of printer mechanisms. A linear motor is a motor wherein the axis of movement of the movable element of the motor is rectilinear rather than rotary. One example of a linear motor shuttling system designed to shuttle a flexure mounted carriage that supports the print head of a dot matrix line printer is described in U.S. Pat. No. 4,461,984 entitled "Linear Motor Shuttling System" by C. Gordon Whitaker et al., assigned to the assignee of the present application. While linear motor shuttling systems have proven to be substantially superior to the types of shuttling mechanisms described above, particularly in high speed dot matrix line printers, they also have certain disadvantages. The primary disadvantage of linear motor shuttling systems is their size and cost.
The present invention is directed to providing a shuttle drive for reciprocably mounted carriages, particularly flexure mounted carriages, that overcomes the disadvantages of prior shuttle drive mechanisms. In particular, the present invention is directed to a shuttle drive mechanism for reciprocably mounted carriages that is highly accurate, inexpensive and relatively small in size.